Comment by duskwuff
I have a hard time imagining what kind of "side project" would be making use of the Amazon Selling Partner API. It all revolves around managing listings and sales through the Amazon storefront - if you've got a use case for that, it's probably for something serious.
This is true of no end of APIs. It's another cost that would be difficult to be borne by a bootstrapped startup still finding PMF and early traction.